Obstetric and perinatal outcomes in women with previous breast cancer: a nationwide study of singleton births 1973-2017.

study questionWhat are the obstetric and perinatal outcomes in births to breast cancer survivors compared to women without previous breast cancer? SUMMARY ANSWER: Women who conceived during the first 2 years following a breast cancer diagnosis had a higher risk for preterm birth, induced delivery, and cesarean section, while no increased risks were observed in births conceived later than 2 years after a breast cancer diagnosis. WHAT IS KNOWN ALREADY: A recent meta-analysis found higher risks of cesarean section, preterm birth, low birthweight, and small for gestational age in pregnancies among breast cancer survivors. Less is known about rarer outcomes such as pre-eclampsia or congenital malformations. STUDY DESIGN SIZE DURATION: We conducted a population-based matched cohort study including all breast cancer survivors who gave birth to singletons 1973-2017 in Sweden, identified through linkage between the Swedish Cancer Register, the Medical Birth Register, and the National Quality Register for Breast Cancer. PARTICIPANTS/MATERIALS SETTINGS

methodsEach birth following breast cancer (n = 926) was matched by maternal age at delivery, parity, and calendar year at delivery to 100 births in a comparator cohort of women (n = 92 490). Conditional logistic and multinomial regression models estimated relative risks (RR) with 95% CI. Subgroup analyses by time since diagnosis and type of treatment were performed. MAIN RESULTS AND THE ROLE OF CHANCE: Previous breast cancer was associated with higher risks of induced delivery (RR; 1.3, 1.0-1.6), very preterm birth (RR; 1.8, 1.1-3.0), and planned preterm birth (RR; 1.6, 1.0-2.4). Women who conceived within 1 year after breast cancer diagnosis had higher risks of cesarean section (RR; 1.7, 1.0-2.7), very preterm birth (RR; 5.3, 1.9-14.8), and low birthweight (RR; 2.7, 1.4-5.2), while the risks of induced delivery (RR; 1.8, 1.1-2.9), moderately preterm birth (RR; 2.1, 1.2-3.7), and planned preterm birth (RR; 2.5, 1.1-5.7) were higher in women who conceived during the second year after diagnosis. Women who conceived later than 2 years after breast cancer diagnosis had similar obstetric risks to their comparators. LIMITATIONS REASONS FOR CAUTION: As information on the end date of treatment was unavailable, the time between the date of diagnosis and conception was used as a proxy, which does not fully capture the effect of time since end of treatment. In addition, treatments and clinical recommendations have changed over the long study period, which may impact childbearing patterns in breast cancer survivors. WIDER IMPLICATIONS OF THE

findingsRisks of adverse obstetric outcomes in breast cancer survivors were confined to births conceived within 2 years of diagnosis. As family building holds significance for numerous young breast cancer patients, these findings are particularly important to inform both breast cancer survivors and clinicians about future reproductive outcomes. STUDY FUNDING/COMPETING INTERESTS: This work was supported by the Swedish Cancer Society (grant number 22-2044 Pj A.L.V.J.), Karolinska Institutet Foundations (grant number: 2022-01696 F.E.L., 2022-01559 A.L.V.J.), and the Swedish Research Council (grant number: 2021-01657 A.L.V.J.). K.A.R.-W. is supported by grants from the Swedish Cancer Society (20 0170 F) and the Radiumhemmets Research Foundations for clinical researchers 2020-2026. The authors declare that they have no conflicts of interest. TRIAL REGISTRATION NUMBER: N/A.